Former red-shirt leader Jatuporn Prompan accused the Pheu Thai Party of stalling the push for constitutional amendments, saying the ruling party is deliberately dragging its feet to prolong its stay in power.
During a livestream on Facebook, Mr Jatuporn blasted the party for "lacking enthusiasm" in the charter amendment process.
He said Pheu Thai wants to delay the charter amendment process because the current constitution allowed the party to form a ruling coalition and get its candidate, Srettha Thavisin, named the nation's prime minister.
The co-leader of the Kana Lomruam Prachachon ("Melting Pot Group") said if the government were truly serious about rewriting the constitution, various issues relating to the amendment process would have been cleared by now.
"The question is whether or not Pheu Thai really needs to amend the charter now because the charter's contentious clauses, including the one which empowers the senate to co-elect the prime minister, will expire in May next year anyway," he said.
"The amendment is a game [to them]. If they were serious about it, it would have been successfully implemented because senators will support the push, and the opposition is unlikely to obstruct," he said.
According to Mr Jatuporn, as the charter amendment requires support from one-third of the Senate and 20% of the opposition camp, the referendum is not as complicated as it appears to be.
He asked the public to watch out for political changes when the Senate's term expires in May next year, as it would take place around the same time as crucial deliberations on issues such as constitutional amendment, examination of the budget bill and the digital wallet scheme.
When asked about the amnesty bill sponsored by the Move Forward Party, he said the proposal was divisive.
Some were sceptical that it would benefit those convicted of lese majeste, while others feared it would only benefit those affiliated with jailed former prime minister Thaksin Shinawatra.
Move Forward Party list-MP Rangsiman Rome on Monday defended the bill, saying political amnesty is needed to ensure justice for those who were persecuted for expressing their political views.
He insisted that the amnesty bill was not intended to benefit any particular group of protesters and that those who meet the criteria set out in the bill would be granted amnesty, including red-shirt protesters.
In a related development, Senator Wanchai Sornsiri on Monday disagreed with the government's plan to seek a ruling from the Constitutional Court on the new charter rewrite bid.
He said the court had already ruled if a new charter was to be drafted, so a referendum must be held to ask the people if they agreed.
He added if the government did not want to hold a referendum, it could choose to amend the charter section by section.